I have installed a diff lock actuator on my DII. You have 2 choices: fit the factory-option lever, or fit a solenoid actuator. Discoparts sell the solenoid actuators, and Ashcrofts sell all the bits you need to fit the gear lever.
Ashcrofts' website also tells you from the serial number whether your transfer box has the spigot to fit diff lock, or whether you'll need to modify the transfer box, as not all had the spigot.
By default the DII will disable the TC when diff lock is actuated. You can disable the electronics which does this fairly easily; for instructions see the fitting instructions on Ashcrofts website, which I wrote.
The combination of diff lock and TC works very well; the diff lock takes some of the effort away from the TC and it cuts in a lot less than it normally would.